Radar apparatus

ABSTRACT

A radar apparatus provided with a transmitter for periodic transmission of mutually disjunct groups of N radar transmitter pulses and provided with a receiver for the receipt of echo signals of the groups of radar transmitter pulses. The radar apparatus includes a video processor for processing echoes in a listening time observed between two of the mutually disjunct groups of radar transmitter pulses. By choosing a suitable staggering of the pulses in a group, the target range and velocity may be unambiguously determined.

The invention relates to a radar apparatus provided with a transmitterfor the periodic transmission of mutually disjunct groups of N radartransmitter pulses, with N=4, 5, 6, . . . , a receiver for the receiptof echo signals of the radar transmitter pulses and a video processorfor detecting possible objects on the basis of the received echoes andfor estimating parameters of these objects.

Radar apparatus of this type are well-known in the art. The devices inquestion are radar apparatus in which a listening time is observedbetween two successive radar transmitter pulses to enable the receipt ofecho signals. The drawback of these known radar apparatus is that, fordistant targets, the interval between two pulses shall be chosen to belarge which, however, would preclude the unambiguous determination of atarget's radial velocity.

The inventive radar apparatus meets this drawback to a significantextent and is characterized, according to an aspect of the invention, inthat the video processor is designed to process echoes in a listeningtime observed between two groups of radar transmitter pulses. For onetarget, N echoes are received on the basis of which both target rangeand radial velocity can be determined unambiguously.

A favourable embodiment of the radar apparatus according to theinvention is characterized in that the video processor is provided withmemory means for storing a row of complex video strengths determined bythe receiver. This row can be searched for a pattern of echo signalsthat corresponds to the transmitted pattern. Besides, by considering thephase shift of the N echo signals, it is possible to determine theDoppler frequency and, thus, the radial velocity of the targetassociated with the echo signals.

A further favourable embodiment of the radar apparatus according to theinvention is characterized in that the video processor is provided witha digital filter for processing a continuous subrow from the row ofvideo strengths. This entails the advantage that the subrow may beconsiderably shorter than the row, which allows the use of a digitalfilter with a limited number of inputs. More precisely: the subrow shallbe so long to enable the simultaneous processing of the N echo signalsreflected by a point target. It will then suffice to pass the row ofcomplex video strengths through the digital filter in order to obtain asubstantially optimal detection.

A very favourable embodiment according to a further aspect of theinvention is characterized in that the digital filter comprises aFourier transformer or an equivalent linear transformer. Not only doesthis enable a substantially optimal detection but also renders itpossible to simultaneously determine, on the basis of the phase shift ofthe N echo signals, the Doppler frequency and, thus, the radial velocityof a target.

A further favourable and simple embodiment of the radar apparatusaccording to the invention is characterized in that within a group, thepulses are mutually phase-coherent and a time interval between twopulses is always a multiple of a unit of time.

A favourable embodiment according to a further aspect of the inventionis characterized in that the: transmitter is designed to position thepulses in a group in order of time in such a manner that for a pointtarget the Fourier transformer delivers an output signal for at leastsubstantially one subrow.

A favourable embodiment according to a further aspect of the inventionis characterized in that out of all possible suitable pulse patterns, apulse pattern is selected for which in case of a point target, theFourier transformer delivers an output signal with minimum side lobes.

The invention furthermore relates to a method for operating a radarapparatus according to which groups of N radar transmitter pulses areperiodically transmitted, with N=4, 5, 6, . . .

According to an aspect of the invention, the inventive method ischaracterized in that, in a listening time between two groups of radartransmitter pulses, the received radar echo signals are applied to avideo processor to estimate parameters of a possible detected target.

A favourable realization of the inventive method is characterized inthat the received radar echo signals are digitized and sequentiallypassed through a digital filter with at least N inputs for combining theN echo signals reflected by a target.

According to a further aspect of the invention, a favourable realizationof the method is characterized in that within a group, the radartransmitter, pulses are transmitted non-equidistantly in time, in whichcase the radar transmitter pulses within a group are positioned suchthat at least for a point target, the digital filter delivers outputsignal at least substantially once.

A favourable realization of the method according to a further aspect ofthe invention is characterized in that the received radar echo signalsare digitized and sequentially applied to a Fourier transformer with Ninputs and M outputs, where M>>N. The velocity of a target can then bedetermined at the same time. The radar transmitter pulses within a groupare preferably positioned such that at least for a point target, theFourier transformer delivers an output signal with minimum side lobes.

A favourable realization according to a further aspect of the inventionis characterized that for each transmitted group and each subrow apredetection is generated for at least the strongest output signal ofthe Fourier transformer or an equivalent transformer. A detection isgenerated if at least P practically identical predetections aregenerated for the same subrow from Q consecutively transmitted groups,with P=1, 2, . . . , Q=1, 2, and P≦Q.

The invention will now be explained in greater detail with reference tothe following figures, of which:

FIG. 1 represents a block diagram of the inventive radar apparatus;

FIG. 2A represents a possible group of radar transmitter pulsesaccording to the invention;

FIG. 2B represents an autocorrelation function for this group;

FIG. 3A represents a possible video processor for stationary targets;

FIG. 3B represents a possible video processor for targets of knownvelocity;

FIG. 4 represents a possible video processor for targets of randomvelocity;

FIG. 5 represents a possible spectrum of a target.

FIG. 1 represents a block diagram of a possible embodiment of theinventive radar apparatus. A transmitter 1 generates groups of radartransmitter pulses that are transmitted via a transmitting antenna 2.Radar echo signals are received by a receiving antenna 3 and areprocessed in a receiver 4 to yield a complex digital video signal.Generally, it is also possible to use one combinedtransmitting/receiving antenna under the application of a T/R unitwell-known in the art. The video signal produced by receiver 4 isapplied to a video processor 5 where it is temporarily stored in memorymeans 6 to await further processing. The contents of memory means 6 isfiltered by a digital filter 7 whose output is connected to a thresholdcircuit 8. In case of a crossing of a preestablished threshold, circuit8 will generate a predetection, which possible predetections arecombined in a combiner 9 for consecutively transmitted groups of radartransmitter pulses to yield a detection that is applied to an output 10for further processing.

FIG. 2A represents a possible group of radar transmitter pulsesaccording to the invention. The group shows several characteristicfeatures. Firstly, the interval between two pulses is far shorter thanthe listening time of the radar apparatus; i.e. the time taken by apulse to travel to a pulse a target and return. Secondly, the durationof a pulse is shorter than the interval between two pulses, in this caseT1, T2, T3. Thirdly, the group is chosen such that the autocorrelationfunction of the group shows only one peak. According to the invention,the time intervals T1, T2, T3 are preferably chosen differently.

According to the invention, a group is transmitted and a target echo isprocessed for the entire group simultaneously. To this end, the targetecho is applied to an autocorrelator, which presents the position of thetarget by a peak in the autocorrelation function.

FIG. 2B represents an autocorrelation function for the group shown inFIG. 2A. The position of the pulses has been carefully determined sothat there is only one peak of height four, with all side lobes havingheight one. It will be evident that the number of groups possessing thisfeature is considerable, certainly if T1, T2, T3 can be chosen to belarge. However, the group must not become too long, as this would causethe minimum range of the radar apparatus to become too large. The numberof pulses to be contained in a group should remain within specifiedlimits, as otherwise, the transmitter's maximum duty cycle would beexceeded.

In general, a radar apparatus does not emit four but about ten pulses.This entails the advantage that the main lobe/side lobe ratio of theautocorrelation function improves, which increases the detectionprobability.

FIG. 3A is a detailed representation of a possible digital filter 7 forstationary targets. The filter is applied to memory means 6, dividedinto range quants, that contains a row of complex video strengths asproduced by the radar receiver. In case of only one target, the fourmemory cells 11 a, 11 b, 11 c, 11 d each contain a target strengthhigher than the noise level, originating from the four pulsestransmitted in one single burst. Digital filter 7 is here implemented asan adder circuit which continuously adds target strengths from fourmemory means in accordance with the transmitted pattern. Owing to thecharacteristic feature of the transmitted group, only one output signalthat represents the target range appears at the output of digital filter7. In case of moving targets, the four target strengths show a varyingphase which results in detection losses.

FIG. 3B represents a possible digital filter 7 for targets of knownvelocity, where digital filter 7 is provided with phase-shiftingnetworks 12 a, 12 b, 12 c, 12 d. The networks 12 a, 12 b, 12 c, 12 dhave been chosen such that for one selected velocity, based on thetiming of the pulses within a group, the phase shift occurring as aresult of target motion is compensated. This restores/reactivates theoriginal autocorrelation function and elimates detection losses.

FIG. 4 represents a possible video processor for targets of randomvelocity. Digital filter 7 is implemented as a Fourier transformer whichis applied to memory means 6. Because the group shown in this examplehas a length of 16 range quants, use is made of a 16-point Fouriertransformer of which only the outputs 0, 5, 11 and 15 are connected tothe corresponding range quants, which ensures an optimal summation ofthe received signal. The other inputs remain unused. Fully analogous tothe correlator described with reference to FIG. 3A, the correlationsignal in case of stationary targets appears at output 0 of the Fouriertransformer. Echo signals of moving targets emerge at the other outputsof Fourier transformer 7 in order to allow the determination of targetposition and velocity.

According to the invention, the output signal of Fourier transformer 7is fed to threshold circuit 8. In threshold circuit 8, a threshold valuecan in a manner known in the art be determined per range quant from the16 input values; also, it can be verified whether one input valuesignificantly exceeds the threshold, which would indicate the presenceof a target in this range quant, after which a predetection can begenerated. Subsequently, a detection probability and a false-alarm ratecan in an obvious manner be established by a person skilled in the art.

It is also possible to generate detect ions without the introduction ofa threshold value. This method is based on the assumption that in thepresence of a target there is one maximum in the spectrum. In theunlikely event of two maxima occurring, one of these maxima isarbitrarily designated as the maximum. Per transmitted group, thresholdcircuit 8 remembers for each range quant which output of Fouriertransformer 7 showed the highest value. Assuming that in the absence ofa target there is no preference for a certain output, it can, forconsecutively transmitted groups per range quant be verified in combiner9, if a certain output is statistically significantly more often thehighest. If such is should be the case, a detection is supplied tooutput 10.

The input values to Fourier transformer 7 are in fact unweighed, whichmay give rise to the occurrence of side lobes in the Doppler domain inthe output signal. It is therefore recommendable to verify for thefinite number of possible groups of pulses which group produces thesmallest side lobes and subsequently to use this group.

FIG. 5 represents a possible spectrum of a target as it appears at theoutput of Fourier transformer 7. In this simulation, 11 pulses aretransmitted at a radar frequency of 1400 Mc/s and a target is simulatedat a velocity of 0 m/s. The horizontal scale is presented in m/s; thefigure reveals the extremely high folding speed of 16000 m/s.

What is claimed is:
 1. A radar apparatus comprising: a transmitterconfigured to periodically transmit mutually disjunct groups of N radartransmitter pulses, with N=4, 5, 6, . . . ; a receiver configured toreceive echo signals of the radar transmitter pulses; and a videoprocessor configured to detect possible objects based on the receivedechoes and to estimate parameters of the possible objects, wherein thevideo processor is configured to simultaneously process echoes from anentire group of said mutually disjunct groups in a listening timeobserved between two of the mutually disjunct groups of radartransmitter pulses.
 2. The radar apparatus as claimed in claim 1,wherein the video processor includes a memory configured to store a rowof complex video strengths determined by the receiver.
 3. The radarapparatus as claimed in claim 2, wherein the video processor includes adigital filter configured to process a continuous subrow from the row ofvideo strengths.
 4. The radar apparatus as claimed in claim 3, whereinthe digital filter comprises a Fourier transformer or an equivalentlinear transformer.
 5. The radar apparatus as claimed in claim 4,wherein within a group, pulses are mutually phase-coherent and a timeinterval between two pulses is always a multiple of a unit of time. 6.The radar apparatus as claimed in claim 5, wherein the transmitter isconfigured to position the pulses in a group in order of time such thatfor a point target the Fourier transformer delivers an output signal forat least substantially one subrow.
 7. The radar apparatus as claimed inclaim 6, wherein out of all possible suitable pulse patterns, a pulsepattern is selected for which in case of a point target, the Fouriertransformer delivers an output signal with minimum side lobes.
 8. Amethod for operating a radar apparatus comprising: transmittingperiodically mutually disjunct groups of N radar transmitter pulses,with N=4, 5 ,6, . . . ; and applying simultaneously received radar echosignals from an entire group of said mutually disjunct groups to a videoprocessor to estimate parameters of a possible detected target during alistening time between two of the mutually disjunct groups of radartransmitter pulses.
 9. The method as claimed in claim 8, furthercomprising: digitizing the received radar echo signals; and sequentiallyapplying the digitized echo signals to a digital filter comprising atleast N inputs.
 10. The method as claimed in claim 9, further comprisingtransmitting the radar transmitter pulses non-equidistantly in time. 11.The method as claimed in claim 10, further comprising positioning theradar transmitter pulses within a group such that at least for a pointtarget, the digital filter delivers an output signal at leastsubstantially once.
 12. The method as claimed in claim 11, furthercomprising: digitizing the received radar echo signals; and sequentiallyapplying the digitized echo signals to a Fourier transformer with Minputs, where M>>N.
 13. The method as claimed in claim 12, furthercomprising positioning the radar transmitter pulses within a group suchthat at least for a point target, the Fourier transformer delivers anoutput signal with minimum side lobes in the frequency domain.
 14. Themethod as claimed in claim 13, further comprising generating apredetection per transmitted group and per subrow for at least astrongest output signal of the Fourier transformer.
 15. The method asclaimed in claim 14, further comprising generating a detection if atleast P identical predetections are generated for the same subrow from Qconsecutively transmitted groups, wherein P=1, 2, . . . , Q=1, 2, . . ., and P≦Q.